The present invention relates to the art of injection molding hollow plastic articles especially hollow plastic articles which are blow molded into containers such as oriented hollow plastic containers and to the art of injection molding large capacity shipping containers.
A high rate of production is important in commercial operation involving the injection molding of plastic articles. The rate at which an injection molding cycle can produce plastic articles is limited by the time it takes to cool the articles to permit damage-free post molding handling. When forming a relatively thick walled part, the major portion of the cycle time for injection molding is the mold cooling time--the time during the cycle after the mold has been filled and packed with resin and before the part is ejected from the mold.
Examples of thick walled parts are preforms, molded in polyethylene terephthalate (PET), with wall thicknesses on the order of 0.060 to 0.080 inches and four and five gallon shipping containers, molded in high density polyethylene (HPDE), with wall thicknesses of 0.0035 to 0.050 inches. Preforms typically have a total molding cycle of 19 to 30 seconds and shipping containers have a cycle of 23 to 30 seconds. The cooling time portion of these cycles is typically one third to one half of the total cycle, and in some cases can be higher.
The processing of PET resin forms the chemical acetaldehyde (AA) which can impart an undesirable taste to the contents of containers molded from PET. The amount of acetaldehyde generated during processing bears a linear relationship with the time taken to process and an exponential relationship with the temperature of the PET during processing. Techniques which significantly reduce the amount of acetaldehyde generated during PET processing are quite desirable.
One known injection molding technique is shown in U.S. Pat. No. 3,384,930 to Rees. A machine capable of interchanging mold cores is used in this technique. The machine is used to injection mold closures having internal threads which have to be unscrewed from the mold cores. By being able to remove the first set of cores with the molded closures and inserting a second set of cores into the mold, the first set of closures can be unscrewed during the same time that a second set of closures are being molded on the second set of cores. In this technique, cooling is performed during the complete cycle before the mold is opened. As a result, the total cycle time is not reduced. Another disadvantage of this approach is that the closures must be sufficiently cooled and strong enough before unscrewing can begin. The time for the unscrewing operation is determined by the pitch and number of turns of the thread required to disengage the closures from the cores. Consequently, the unscrewing time could be as long as the complete molding cycle including cooling time.
U.S. Pat. No. 4,439,133 to Rees et al. illustrates another example of a machine capable of interchanging mold cores. The machine uses a four faced turret to carry mold cores for forming PET preforms. Each of the four faces can be inserted into the mold to produce preforms. During the same time one set of cores is forming preforms, the other sets are in locations for cooling and ejecting the previously molded parts. One disadvantage with a turret arrangement is that a large machine opening stroke is required for each cycle. This extends the cycle time and offsets gains achieved by simultaneous performance of the cooling operation. A second disadvantage with a turret system is that the turret mechanism is very complex since all four faces must carry a core half and its associated ejection mechanism. This additional complexity increases cost and the risk of a mechanical failure resulting in downtime.
Some injection molding techniques require a cooling time which exceeds the remaining portion of the cycle time. As a result, parts formed during these techniques will not be completely cooled before the next set of molded parts is ready to begin cooling and/or before the first set of cores is required to be inserted into the mold to begin the next molding cycle.
U.S. patent application Ser. No. 31,655, filed Mar. 30, 1987 to Schad et al., which is a division of U.S. patent application Ser. No. 733,969, filed May 14, 1975, which is in turn a continuation-in-part of U.S. Pat. No. 4,522,581, illustrates an example of post cooling of preforms. In this approach, a tapered cooled tube is mounted on a robot tooling plate for transporting the partially cooled preform out of the mold while continuing cooling.
A second example of post cooling preforms is disclosed in U.S. patent application Ser. No. 29,166, filed Mar. 23, 1987, to Delfer III. In this approach, a cooled storage plate is used to store and cool sequential cycles of preforms as they are ejected in a partially cooled condition from the mold. The disadvantages of this technique include the required length of the opening stroke and the time required to permit insertion and retraction of the cooled storage plate.
Accordingly, it is an object of the present invention to provide an injection molding apparatus capable of significantly reducing molding cycle time and thereby increase the throughput of the plastic material being processed.
It is a further object of the present invention to provide an apparatus as above capable of significantly reducing the amount of acetaldehyde generated during PET processing by reducing the time the PET is in an elevated temperature state in the apparatus and mold.
It is yet a further object of the present invention to Provide an injection molding apparatus as above which achieves a higher production rate without increasing the number of mold cavities
It is yet another object of the present invention to provide an injection molding apparatus as above capable of simultaneously performing cooling of a set of plastic articles and the next sequential molding cycle.
It is still another object of the present invention to provide an injection molding apparatus as above capable of continuous cooling after the molded parts have been ejected from the mold core.
These and other objects and advantages will become more apparent from the following description and drawings.